The invention relates to a method for laying a paving mat of paving material on a planum with a paving screed being towed floatingly in travelling direction on the paving material by a road paver, the paving mat comprising a lane of a determined width and at least one sideward slope, the slope being inclined downwardly from a transition into the lane, the paving screed including a base screed and at least one extension screed which is mounted at the front side or the rear side of the base screed and is slidable lateral to the travelling direction substantially parallel to the base screed for varying a working width, the base screed and the extension screed having respective sole plates, the sole plate of the extension screed being inclined laterally at the angle of the slope relative to the sole plate of the base screed, according to which method in the case of a variation of the working width by sliding the laterally inclined extension screed relative and parallel to the base screed and/or in the case of a variation of the angle of the slope by laterally tilting the sole plate of the extension screed relative to the sole plate of the base screed, in order to maintain the predetermined lateral position of the transition stationarily. The sole plate of the extension screed is adjusted in height direction relative to the base screed by means of at least two height adjustment assemblies which are distant from each other in sliding direction of the extension screed. The method is characterised by the following steps:    during or after setting the lateral position a height difference is measured substantially perpendicular to the planum between a fixed measuring location at the base screed and a location along a reference line fixed at the extension screed, which reference line is parallel to the laterally inclined extension screed and the slope,    the measured height difference is stored as a target value,    a deviation of the height difference from the target value caused by the variation is measured while the paving material is laid,    and the deviating height difference is corrected to the target value exclusively by an automatic height adaptation of the sole plate of the laterally inclined extension screed by actuating the height adjustment assemblies corresponding to the measured deviation.
According to the method known from U.S. Pat. No. 5,568,992 A both height adjustment assemblies of the extension screed, which height adjustment assemblies are provided for adjusting both the height position of the sole plate of the extension screed and the angle of the slope, are actuated with complicated calculation operations for which linear position signals of both height adjustment assemblies and of a sliding drive of the extension screed are processed in combination with input target positions in order to maintain the lateral position of the transition with respect to the base screed stationary. When carrying out these calculation operations even signals of angle sensors are considered which detect the angle of attack of the paving screed and the angle of the inclination of the road paver relative to the planum. As, according to the known method, simultaneously a plurality of information has to be evaluated and processed, the regulating system used to carry out the method is complicated and prone to failure.
According to a method known from DE-U-92 11 854 the sole plate of the extension screed is automatically maintained aligned with the sole plate of the base screed at a respective correct height position corresponding to the respective angle of attack of the paving screed relative to the planum, such that longitudinal steps are avoided in the surface of the paving mat.
According to the method known from DE-27 09 435 C the height adjustment assemblies provided in the extension screed are used both for a height adjustment and for adjusting the lateral inclination of the sole plate of the extension screed. Adjustments of the lateral inclination are executed for forming crown profiles of the paving mat surface. The height adjustment assemblies contain screw spindles or hydro-cylinders which can be remotely actuated independent from the angle of attack of the paving screed.
During the production of the paving mat continuously extending over the working width of the paving screed the working width is varied by sliding the extension screed along the base screed. The angle of attack of the paving screed relative to the planum influences the thickness of the paving mat and can be varied or is varied. A variation of the angle of attack of the paving screed needs to correspondingly adapt the height position of the rear edge of the sole plate of the extension screed relative to the rear edge of the sole plate of the base screed. A laterally varying mat thickness is adjusted by different height positions of the towing points of the towing bars of the paving screed at the road paver. Among others paving mats frequently laid in Northern America have a lane of predetermined width and at least one sideward laterally inclined slope. In this case the sole plate of the extension screed is inclined laterally to the travelling direction. As in the case of a variation of the working width normally the width of the lane should not vary, it is important to then maintain the lateral position of the transition between the lane and the slope relative to the base screed stationary.
It is an object of the invention to provide a method allowing to reliably maintain the lateral position of the transition with respect to the base screed stationary, and which only needs to process a minimum amount of information.
According to the invention when determining or after determining the lateral position a height difference is measured substantially perpendicular to the planum P and between a fixed measuring member located at the base screed and a location along a reference line at the extension screed, which reference line is parallel to the sole plate of the laterally inclined extension screed and the slope. The height difference is stored as a target value. In the case of a variation of the working width and/or of the angle of the slope a deviation of the height difference from the target value caused by the variation is then measured. The deviating height difference then is corrected to the target value solely by an automatic height adaptation of the laterally inclined extension screed by means of the height adjustment assemblies corresponding to the measured deviation of the sole plate.
According to this method exclusively a height measurement has to be carried out in order to detect the height position of the sole plate of the extension screed relative to the sole plate of the base screed. The lateral position of the transition between the lane and slope first is fixed by e.g. adjusting the height position and/or the lateral inclination of the sole plate of the extension screed so that the lateral position will be situated at a desired location relative to the base screed. With the help of the fixed lateral position the height difference is measured perpendicular to the planum and is converted to a target value. In the case of a variation of the working width and/or of a variation of the angle of the slope needed while the paving mat is laid height measurements are carried out. Then only a detected deviation from the target value resulting from the variation is used to carry out a height adjustment of the sole plate of the extension screed so that the lateral position is maintained stationarily. This results in a reliable method for maintaining the lateral position of the transition. It is only necessary to obtain, evaluate and process a minimum amount of information, namely only the height difference.
Expediently the height difference and/or the deviation from the target value is measured by a measuring member of a height measuring device, the measuring member being fixed at the base screed. The measuring member measures reliably irrespective of the momentary sliding position of the extension screed at the base screed, and directly delivers the information which is needed for the actuation of the height adjustment assembly in order to automatically maintain the lateral position stationary.